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Towards high resolution ceramic series for production site studies: the case of Loron amphorae (Croatia, 1st–3rd c. A.D.)

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Towards high resolution ceramic series for production site studies: the case of Loron amphorae (Croatia, 1st–3rd c. A.D.)
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  Machut et al. Herit Sci (2015) 3:21 DOI 10.1186/s40494-015-0050-5 RESEARCH ARTICLE  Towards high resolution ceramic series for production site studies: the case of Loron amphorae (Croatia, 1st–3rd c. A.D.) Pierre Machut 1*† , Ayed Ben Amara 1† , Nadia Cantin 1† , Rémy Chapoulie 1† , Nicolas Frèrebeau 1† , François-Xavier Le Bourdonnec 1† , Yolande Marion 2†  and Francis Tassaux 2† Abstract   Background:  From the 1st to the 3rd centuries A.D., the territory of Istria (present-day Croatia) was a prominent area of olive oil production. Archaeologists have identified the so-called Dressel 6B amphora as the main container used in the transport and trade of this oil. So far archaeology and epigraphy have helped identify probable sources of production and two workshops in Istria, and have also allowed the reconstruction of main trade routes through the identification of stamped amphorae. However, much less is known about the organization of the production of these ceramic containers, which may have been conducted on an almost industrial scale. This preliminary study demon-strates how chemical analysis using energy-dispersive X-ray fluorescence spectrometry (EDXRF) may address this question and provide new answers and avenues of inquiry. Results:  Our study of 53 amphorae, produced during two important periods in the history of the workshop of Loron, has evidenced that even in chronologically closely related contexts of production chemical analysis could significantly separate samples according to production groups. Conclusions:  This result constitutes a very important first step towards a thorough study of the entire production history of the workshop, involving the creation of a comprehensive geochemical reference group comprising several hundred samples. This in turn will enable us to address an array of hypotheses concerning the economy and ecology of this production site, which are briefly presented in the article. The application of this methodology to the whole corpus of Istrian amphorae is then advocated in light of this study. © 2015 Machut et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the srcinal author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Background Within the field of ceramic characterization, comprehen-sive studies of workshop production sites usually require considerable analytical work, which exceeds by far the scope of most archaeological science research projects. Pollard and Heron pointed out the implications of this issue in 1996: “most kiln sources are characterized by at best a hundred analyses, probably covering a range of “qualities” of vessels produced, a range of vessel types, and, possibly, a time span of tens or even hundreds of years. Essentially the archaeological chemist is relying on the quality control procedures in force in antiquity to ensure that the sample is representative of the range of composi-tions produced!” [1]. Any study aiming to precisely assess the range of compositions produced in a specific loca-tion should thus address this question of representative-ness first and foremost. Sample selection is a crucial step in this regard, and appropriate sampling should include a suitable number of samples of each vessel type for every production group recorded at a workshop site. Conse-quently, hundreds of samples potentially need to be pro-cessed in order to answer even the simplest archaeological questions with minimal confidence. Although such high-resolution studies are—for obvious reasons—scarce, we advocate that attempting to apply it to an appropriate case study would yield significant results. Open Access *Correspondence: pierre.machut@u-bordeaux-montaigne.fr † All authors contributed equally to this work  1  IRAMAT-CRP2A, UMR 5060 CNRS, Université Bordeaux Montaigne, Pessac, FranceFull list of author information is available at the end of the article  Page 2 of 7Machut et al. Herit Sci (2015) 3:21 In order to demonstrate this assumption, we chose two well-defined production groups—according to archaeo-logical criteria—from the Loron amphora workshop site, dating from the Roman era. Subsequently, we applied quantitative chemical analysis by energy-dispersive x-ray fluorescence spectrometry (EDXRF) and statistical data treatment on a significant number of those samples.From the 1st to the 3rd centuries A.D., the territory of Istria (present-day Croatia) was a prominent area of olive oil production. is oil of great repute was exported in the so-called Dressel 6B (Dr 6B) amphorae towards northern Italy and the Danubian provinces, along major trade roads in the Empire (Figure 1). Pro-duction during the 1st century was dominated by the workshops of Fažana in the southern part of the pen-insula, on the territory of Pola (modern-day Pula), and Loron, on the territory of Parentium (modern-day Poreč) (Figure 2). Initially, they were private proper- ties controlled by aristocratic families, but then both workshops became part of the imperial domain under the Flavians. At Loron, the history of ownership is well documented through extensive stamping of the ceramic containers.Two decades of intensive research have resulted in a precise picture of the ceramic categories produced at Loron and of the trade circuits of Dr 6B amphorae [2]. However, archaeological and epigraphic investigations fail to answer precise questions about the actual produc-tion process established in the workshop. Despite much speculation about the organization of production a lot of questions remain, since only two workshops at Loron and Fažana have been located and partially excavated. Moreover, our knowledge of epigraphic sources is lim-ited to only a few examples in the case of some lesser-known stamps while most of the ceramic material is still untapped due to lack of inscriptions.When dealing with such large structures, the output of which would have been enormous for preindustrial times, the aspects of technology and standardisation must also be considered in order to address the actual questions of historical significance. Namely, understand-ing the inner workings of Roman economy through the specific case of olive oil trade.e two production groups chosen for this study are separated in time by a gap of at most 50 years. Each one corresponds to an important step in the history of the workshop: the productions of Sisenna, founder and first owner of the site, and Domitian, first owner of imperial status. e owners are clearly identified by the amphora stamps bearing their names. Dating of each production group comes mainly from the identification of these characters in other sources such as epigraphy.EDXRF was chosen for its versatility and ubiquity in archaeological ceramic paste composition studies, for which its potential has been demonstrated many times. Any attempt to highlight patterns in a dataset for such a restricted scale (locally and chronologically) needs to make use of robust data analysis methods; thus our results were submitted to exploratory statistical analysis. Results Quantitative results were obtained for 9 major, minor and trace elements measured by EDXRF in all 53 samples (cf. “Methods” below). Elemental composition ranges for both production groups are reported in Table 1. Several observations can be made at once from a cursory exami-nation of the results. Firstly, ceramic pastes are calcare-ous, with a mean of 14.01 wt.% CaO in the case of Sisenna and 10.41 wt.% CaO in the case of Domitian. Moreover, the iron content is significant, with values higher than 6 wt.%. Low relative standard deviations show that each group is internally quite homogeneous.Both individuals and variables scatterplots of the results of principal component analysis are displayed in Figures 3, 4. e first two components, accounting for almost 87% of explained variation, show a clear tendency for values to plot according to production group srcin, even though clustering is not optimal at this point.is exploratory approach already provides us with a promising result, since patterns readily emerge. In order to improve this result and achieve better grouping, dis-criminating elements were chosen from the variables Figure 1  Picture of a Dr 6B amphora.  Page 3 of 7Machut et al. Herit Sci (2015) 3:21 scatterplot of the PCA and plotted in bivariate graphs of log-ratios of elements. As is shown in the exam-ple of Figure 5, excellent group separation is achieved when plotting log-ratios of K 2 O and Sr with SiO 2 . e 95% probability ellipses show no overlap of values. Each cluster is strictly related to one production group as defined by archaeological criteria. is result thus indi-cates a possible srcin of historical significance to these patterns. Discussion e main question that needs to be addressed is which factors can explain these distinct chemical signatures, supported by accurate data on a significant number of samples for each group. As of now, several possible explanations can be hypothesized, which will be further addressed in future research.e first level of explanation we can readily suggest is a naturally occurring variation within the clayey material Figure 2  Map of Istria showing places cited in the text and main geological units. Table 1 Composition ranges for Sisenna and Domitian production groups Major and minor elements are given in mass percentages and expressed as oxides; trace elements are given in parts per million. Al 2 O 3  SiO 2  K  2 O CaO TiO 2  Fe 2 O 3  Rb Sr Zr Average Sisenna14.2556.121.7714.010.836.01119278156St. dev.0.521.400.102.070.040.29122611%St. dev.3.662.495.6514.744.654.781097Average Domitian14.8357.562.2110.410.886.38138220153St. dev.0.361.180.081.130.050.27121410%St. dev.2.442.063.6310.896.024.29866  Page 4 of 7Machut et al. Herit Sci (2015) 3:21 used, whether as a result of the use of two separate clay sources, or due to a high local chemical variation of one single source. Whatever the case, the srcin of the raw material would have been discrete, since the two chemi-cal groups are internally quite homogeneous. When examining the geochemical data, what can be noted is the fact that the observed difference does not refer to SiO 2  or Al 2 O 3  contents but mainly to K 2 O and CaO contents. Depending on whether this difference is related to the elements’ presence in the clay matrix or inclusions, we would interpret it differently. For example, different con-tents of K 2 O in the clay matrix might be associated with a different proportion of illitic clays in the two sources of raw materials used; in inclusions, K 2 O could be related to an abundance of micas or feldspars. Differences in CaO content might be associated with different natural occur-rence of calcareous inclusions, but it could also srcinate from technological choices made by the ancient potters.us, a second level of explanation relates to the tech-nology of amphora production in the workshop and its evolution through time. Clay bodies are seldom made from a naturally occurring single source of clay sediment. Sometimes quite numerous and complex paste prepa-ration steps are introduced in the chaîne opératoire  of the ceramic product. Amphora production is generally regarded as involving minimal paste preparation since the quantities of raw materials needed to ensure a large output would be considerable. Potters are thus supposed to select the most convenient clay material available i.e. one that needs little or no preparation. Nevertheless, this rather simplistic view should not dismiss the fact that addition or removal of non-plastics and clay mixing might have taken place, sometimes in sizeable propor-tions, which would greatly affect chemical results. is could also explain observed differences in K 2 O and CaO contents. Figure 3  PCA analysis: individuals scatterplot of PC1 and PC2. Blue dots  Sisenna production group; red triangles  Domitian production group. Figure 4  PCA analysis: variables scatterplot of PC1 and PC2. Figure 5  Biplot of log-ratios of K  2 O/SiO 2  and Sr/SiO 2 . Blue dots  Sisenna production group; blue ellipse  95% probability ellipse for Sisenna production group; red triangles  Domitian production group; red ellipse  95% probability ellipse for Domitian production group.  Page 5 of 7Machut et al. Herit Sci (2015) 3:21 A third level of explanation stems also from human decisions rather than from natural processes, but is related to the larger scale of economic and political spheres. It is possible that the source used at the time of Sisenna was exhausted when Domitian became the owner of the workshop or even that larger social, politi-cal or economic factors came into play—for instance, if change of ownership resulted in the availability of a new source of raw material located on imperial land. How -ever, these hypotheses cannot be confirmed by the data so far.For the moment, none of these explanations can be chosen with certainty above the others. However, this doesn’t mean we will not be able to elaborate more solid hypotheses in the future. When every production group will have been sampled and analyzed in the same way (totaling to at least 300 samples in the case of Loron), we will hopefully be able to reconstruct the choices made regarding raw material selection and paste prepa-ration during the whole documented activity of at least three centuries. It will be very interesting to see if com-parable patterns emerge, especially for those production groups which lie between Sisenna and Domitian. e present data also need to be supplemented with further methods of analysis, particularly dealing with the min-eralogy of ceramic bodies and also by the consideration of raw materials collected in the surrounding area of the workshop.e recipes chosen by the potters to produce those amphorae are the only remaining trace of numerous fac-tors: choice and preparation of raw materials, the natu-ral availability and variability of those materials, but also how the above varied through more than two centuries of recorded activity, with evidence of changes in owner-ship and trade of the final product. More specifically, it is interesting to investigate the effects of the shift of owner-ship from private to imperial status on the organization of production.Amphorae record every step of their production and use history down to the molecular level. us the inves-tigation of chemical signatures is a necessary first step towards addressing questions of fingerprinting, prov-enance, and technology. e question of provenance of raw materials is certainly not a trivial one in the case of Loron, since the workshop is located in a particular geological context. e Istrian peninsula is composed of Upper Jurassic to Cretaceous carbonate deposits in its southern and western part, and Cretaceous to Paleo-gene carbonate and clastic sequences, overlain by Eocene foraminiferal limestones, transitional beds and flysch deposits) in its eastern and northeastern part (Figure 2). Karstic and weathering processes during the Neogene and the Quaternary have resulted in the formation of different types of sediments and soils. e most notewor-thy of these is terra rossa, a red soil of low calcium con-tent, typical of the Mediterranean climate, found either in karst depressions or as discontinuous surface layers overlying the carbonate plain of southern and western Istria (called quite evocatively “Red Istria”) [3, 4]. e Loron workshop is located in this part of the peninsula and its immediate surroundings abound with terra rossa soils but lack other types of clayey sediments, especially calcareous ones, which would be compatible with the ceramic production identified in the workshop. Access to raw materials would therefore have presented a major issue for ancient potters, especially because significant quantities would have been needed to support large scale production. A similar situation is observed at Fažana, in the southern part of the peninsula. Maria Mange and Tamás Bezeczky have conducted a study of the Fažana amphorae, based on heavy minerals, which suggests that terra rossa might have been used in the production of these containers [5]. Conclusions e results so far are indeed very promising, considering the scope of the planned study. e ability to discriminate statistically two chemical groups relative to the chronol-ogy of production in the workshop is indeed a proof of concept for the capacity of such data to provide exploit-able results even at this scale.Only about half a century or less may separate those two productions, which taken separately are fairly homo-geneous regarding a time span of around 20 years in the case of Sisenna, and 13–15 years in the case of Domitian [2, 6]. erefore there exists the possibility of defining not only a generic reference group for the whole work-shop, but also to establish reference groups for each period of activity in the workshop, which could then be compared to known signatures of other workshops in the same region. Moreover, these groups with high tem-poral resolution could provide a way to securely identify unstamped amphorae (which make up the major part of ceramic finds), and compare to similar productions in other workshops, as for instance in Fažana, which pro-duces vessels of a similar morphology, but most likely of different chemical composition.Of course, significant interpretations will only be drawn from the assessment of all the available data. Epigraphy, for example, has demonstrated the growth of the impe-rial domain at Loron, which could explain—at least par-tially—why a new source of raw material was exploited at this particular time in the history of the workshop. e merit of adding analytical techniques to the arsenal of traditional archaeological methods is to open new ave-nues of inquiry. is appears, indeed, to be the case here,
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